This invention relates generally to the rendering of latent electrostatic images visible using multiple colors of dry toner or developer and, more particularly, to a developer apparatus and method of suppressing the development of the fringe fields of complementary tri-level images.
The invention can be utilized in the art of xerography or in the printing arts. In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a photoconductive insulating surface or photoreceptor. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not struck by radiation.
This charge pattern is made visible by developing it with toner. The toner is generally a colored powder, the powder having been given an electrostatic charge by some means, which adheres to the charge pattern by electrostatic attraction.
The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
The concept of tri-level xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein, the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development system is biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In tri-level xerography, the xerographic contrast on the charge retentive surface or photoreceptor is divided three, rather than two, ways as is the case in conventional xerography. The photoreceptor is charged, typically to 900 v. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged area development, i.e. CAD) stays at the full photoreceptor potential (V.sub.ddp or V.sub.cad, see FIGS. 1a and 1b). The other image is exposed to discharge the photoreceptor to its residual potential, i.e. V.sub.c or V.sub.dad (typically 100 v) which corresponds to discharged area images that are subsequently developed by discharged-area development (DAD). The background areas exposed such as to reduce the photoreceptor potential to halfway between the V.sub.cad and V.sub.dad potentials, (typically 500 v) and is referred to as V.sub.w or V.sub.white. The CAD developer is typically biased about 100 v closer to V.sub.cad than V.sub.white (about 600 v), and the DAD developer system is biased about 100 v closer to V.sub.dad than V.sub.white (about 400 v).
As discussed in U.S. Pat. No. 4,397,264 which relates to a conventional xerographic image development system, conductive magnetic brush (CMB) development and insulating magnetic brush (IMB) development systems suffer from limitations in their abilities to meet the full range of copy quality requirements. Specifically, insulating magnetic brush development systems have difficulty in using a single-developer roller to develop both fine lines and solid areas. In order to optimize solid area development with an insulating developer material, the spacing between the developer roller and photoconductive surface must be made quite small. However, low density fine line development occurs at a larger spacing to take advantage of fringe field development with insulating materials. Insulative developer materials allow development with higher cleaning fields than conductive developer systems so as to minimize background development.
As further discussed in the '264 patent, conductive magnetic brush development systems exhibit low sensitivity to low density lines. Conductive developer materials are relatively insensitive to fringe fields. In order to achieve low density fine line development with conductive developer materials, the cleaning field must be relatively low. This can produce-relatively high background.
The shortcomings of CMB and IMB development systems discussed above with respect to conventional xerography have, heretofore, been present in highlight color xerography as well. In fact, the problem of not being to able to develop low density fine lines with CMB developer in the presence of relatively high cleaning fields favors the use of IMB developer. However, in a tri-level highlight color system the use of IMB developer has been found to be unacceptable. Its use results in the development of fringe fields in a color different from the rest of the image. Thus, for example, in a tri-level system that uses black and red developers of the insulative type, the black images would have a red border around them while red images would have a black border around them.
The development of fringe fields occurs due to the longer range effect on toner particles of the electric field gradients associated with large potential differences at the edges of images. Fringe field development effects are suppressed by CMB development systems because the intrinsic conductivity of the CMB carrier enhances the electric field from charge near the center of the image to give approximately the same attractive force on a toner particle at the center of an image as at the edge. Insulative development systems show this effect weakly or not at all. (See Schaffert, "Electrophotography" (revised edition), Focal Press, 1975, pp. 34-37.)
U.S. Pat. No. 4,868,611 granted to Richard P. Germain on Sept. 19, 1989 relates to a highlight color imaging method and apparatus for forming a single polarity charge pattern having at least three different voltage levels on a charge retentive surface wherein two of the voltage levels correspond to two image areas and the third level corresponds to a background level. Interaction between conductive magnetic brush (CMB) developer materials contained in a developer housing and an already developed CMB image in one of the two image areas is minimized by the use of a scorotron to minimize the potential difference between the charge on an already developed image and the background potential.